The invention relates to a sheet guide device in a rotary printing machine, and more particularly, to an air assisted sheet guide device.
A sheet guide device of such type is disclosed in EP 0 156 173 B1. In that case, the guide device is formed by a plurality of air supply boxes or flow ducts composed of modules and coupled to fans along a continuous guide surface. The air supply boxes have openings or air nozzles, which can be operated optionally to direct forced blown air, or to draw a vacuum, by means of fans.
In addition, it is known for such air supply boxes to be constructed with comb-like ends, which are adjacent to the transfer area of two sheet holding systems. Such a construction is disclosed, for example, in DE 298 17 317 U1.
DE 196 38 311 A1 discloses a method of guiding a sheet and a guide device for a rotary printing machine. The guide device is arranged in the sheet inlet as a suction funnel underneath the tangent point (i.e. the transfer area between two sheet holding systems) of a sheet-carrying cylinder arranged upstream of an impression cylinder. The suction tunnel can be operated only with vacuum. In addition, a sheet guide device that can be operated with mechanical and/or pneumatic means is arranged underneath the sheet-carrying cylinder. One embodiment of a sheet guide device with pneumatic operating means has a comb plate oriented in the direction of the tangent point.
The foregoing arrangements are disadvantageous because in the transfer area between two sheet holding systems of rotary printing machines having sheet-carrying cylinders a sheet has to pass through a transition in a sheet outlet and also in a sheet inlet. There is a transition when the sheet resting on a sheet-carrying cylinder is transferred to a sheet-carrying cylinder arranged downstream of a sheet outlet and is guided by means of a guide device. There also is a transition to the sheet inlet when the sheet, guided by means of a guide device, is transferred to a downstream sheet-carrying cylinder and conveyed resting on that sheet-carrying cylinder.
If a suction effect is permanently present at such transitions, as in DE 196 38 311 A1, the sheet is drawn against a comb plate or guide rods. In that case, the risk increases for smearing or marking the sheet material. This is particularly disadvantageous when sheets are printed on both sides in recto and verso printing.
It is an object of the invention to provide an air assisted sheet guiding device that is adapted for more reliable smear and mark-free sheet conveyance in transition areas between two sheet holding systems having an associated sheet guide device.
Another object is to provide an air assisted sheet guide device as characterized above which can be universally used in recto printing or recto and verso printing.
The invention is carried out by a sheet guide device which has a guide surface and a large number of nozzles that operate on an air blowing/vacuum or venturi principle, to ensure a proper sheet guidance at the sheet transfer area. Optionally, openings fed with blown air or vacuum pressure also can be used in the guide surfaces.
A guide surface of this type is provided adjacent to the transfer area, such as the tangent point of two sheet holding systems, for example gripper systems. In this case, the sheet holding systems are arranged on a sheet-carrying cylinder, for example a transfer cylinder, and a following sheet-carrying cylinder, for example an impression cylinder, or a sheet-carrying cylinder, for example an impression cylinder and a following circulating chain system, for example, a gripper system at the delivery station.
A first advantage of a sheet guide device according to the invention is that it ensures smear-free sheet conveyance in the transfer area between two sheet holding systems and the associated guide device. For this purpose, underneath the transfer area (i.e. tangent point) of the sheet holding systems, a blown air stream is aimed in the direction of the sheet outlet and/or in the direction of the sheet inlet and against the respective sheet. The blown air has low blowing pressure or a low velocity and a high volume flow. A blown air stream of this type ensures smear-free sheet conveyance for sheets created in recto printing and also in recto and verso printing.
It is advantageous in this case that the sheet transferred from the upstream sheet-carrying cylinder to the following sheet-carrying cylinder or to a circulating chain system is carried through the transition to the guide device (sheet outlet) or from the guide device (sheet inlet) without contact with the guide device, and therefore smear-free, by reason of the directed blown air stream. By means of a controllable blown air intensity, virtually ideal tangential guidance of the sheet to the sheet guide device can be effected. The intensity of blown air also can be metered in order to avoid uncontrolled lifting of the sheet. Uncontrolled lifting, for example, leads to problems when the sheet is directed into the next printing zone.
Also advantageous is the fact that, by means of a controllable blown air intensity, the sheet conveyance can be implemented universally in a smear-free manner for various types of sheets and for all modes of operation (i.e. recto printing, recto and verso printing). Therefore, by means of the sheet guide device according to the invention, high printing speeds also can be implemented with a quiet sheet run.